Intravascular ventricular assist device

ABSTRACT

One aspect of an intravascular ventricular assist device is an implantable blood pump where the pump includes a housing defining a bore having an axis, one or more rotors disposed within the bore, each rotor including a plurality of magnetic poles, and one or more stators surrounding the bore for providing a magnetic field within the bore to induce rotation of each of the one or more rotors. Another aspect of the invention includes methods of providing cardiac assistance to a mammalian subject as, for example, a human. Further aspects of the invention include rotor bodies having helical channels formed longitudinally along the length of the body of the rotor where each helical channel is formed between peripheral support surface areas facing radially outwardly and extending generally in circumferential directions around the rotational axis of the rotor.

This application is a divisional U.S. patent application Ser. No.14/171,615, filed Feb. 3, 2014, which application is a continuation ofU.S. patent application Ser. No. 13/196,693, filed Aug. 2, 2011, whichapplication is a divisional of U.S. patent application Ser. No.12/072,471, filed Feb. 26, 2008, which is related to and claims priorityfrom Provisional Patent Ser. No. 60/903,781, filed Feb. 26, 2007 theentirety of all of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates to pumps usable as implantable ventricularassist devices, to components useful in such pumps, and to methods ofusing the same.

In certain disease states, the heart lacks sufficient pumping capacityto meet the needs of the body. This inadequacy can be alleviated byproviding a mechanical pump referred to as a ventricular assist deviceto supplement the pumping action of the heart. It would be desirable toprovide a ventricular assist device which can be implanted and which canremain in operation for months or years to keep the patient alive whilethe heart heals, or which can remain in operation permanently during thepatient's lifetime if the heart does not heal, or which can keep thepatient alive until a suitable donor heart becomes available.

Design of a ventricular assist device presents a daunting engineeringchallenge. The device must function reliably for the desired period ofimplantation. Moreover, blood is not a simple fluid, but instead is acomplex system containing cells. Severe mechanical action can lead tohemolysis, or rupture of the red blood cells, with serious consequencesto the patient. Also, blood in contact with an artificial surface, suchas the surfaces of a pump, tends to clot. While this tendency can besuppressed to some extent by proper choice of materials, surfacefinishes and by administration of anticoagulants, it is still importantto design the pump so that there are no regions within the device whereblood can be trapped or flow is interrupted for relatively prolongedperiods. To provide clinically useful assistance to the heart, thedevice must be capable of delivering a substantial blood flow at apressure corresponding to normal blood pressure. For example,ventricular assist device for an adult human patient of normal sizeshould deliver about 1-10 liters per minute of blood at a pressure ofabout 70-110 mm Hg depending on the needs of the patient.

One type of ventricular assist device or pump uses a balloon. Theballoon is placed within the aorta. The balloon is connected to anexternal pump adapted to repeatedly inflate and deflate the balloon insynchronism with the contractions of the heart muscle to assist thepumping action. Balloon assist devices of this nature have numerouslimitations including limited durability and limited capacity.

As described, for example, in U.S. Pat. No. 6,688,861, a miniatureelectrically-powered rotary pump can be implanted surgically within thepatient. Such a pump has a housing with an inlet and en outlet, and arotor which is suspended within the housing and driven by a rotatingmagnetic field provided by a stator or winding disposed outside of thehousing. During operation, the rotor is suspended within the housing byhydrodynamic and magnetic forces. In such a pump, the rotor may be theonly moving part. Because the rotor does not contact the housing duringoperation, such a pump can operate without wear. Pumps according to thepreferred embodiments taught in the '861 patent and related patents havesufficient pumping capacity to provide clinically useful assistance tothe heart and can be small enough that they may be implanted within theheart and extend within the patient's thoracic cavity. Pumps of thisnature provide numerous advantages including reliability and substantialfreedom from hemolysis and thrombogenesis. However, implantation of sucha pump involves a majorly invasive surgical procedure.

As described, for example, in Nash, U.S. Pat. No. 4,919,647; Siess, U.S.Pat. No. 7,011,620; and Siess et al., U.S. Pat. No. 7,027,875; as wellas in International Patent Publication No. WO 2006/051023, it has beenproposed to provide a ventricular assist device in the form of a rotarypump which can be implanted within, the vascular system, such as withinthe aorta during use. Aboul-hosn et al., U.S. Pat. No. 7,022,100,proposes a rotary pump which can be placed within the aorta so that theinlet end of the pump extends through the aortic valve into the leftventricle of the heart.

A ventricular assist device implanted into the vascular system must beextraordinarily compact. For example, such a device typically shouldhave an elongated housing or other element with a diameter or maximumdimension transverse to the direction of elongation less than about 13mm, and most preferably about 12 mm or less. To meet this constraint,the vascularly-placed ventricular assist devices proposed heretoforeresort to mechanically complex arrangements. For example, the devicedescribed in U.S. Pat. No. 7,011,620 incorporates an electric motor inan elongated housing. The motor drive shaft extends out of the housingand a seal surrounds the shaft. An impeller is mounted at the distal endof the drive shaft outside of the motor housing and within a separatetubular housing. The pump taught in U.S. Pat. No. 7,022,100 consists ofa separate motor using a flexible drive shaft extending through thepatient's vascular system to the impeller, with an extraordinarilycomplex arrangement of seals, bearings, and a circulating pressurizedfluid to prevent entry of blood into the flexible shaft. The arrangementtaught in WO 2006/051023 and in U.S. Pat. No. 4,919,647 also utilizesflexible shaft drives and external drive motors. These complex systemsare susceptible to failure.

Thus, despite very considerable effort devoted in the art heretofore todevelopment of ventricular assist devices, further improvement would bedesirable.

SUMMARY OF THE INVENTION

One aspect of the invention is an implantable blood pump. The pumpaccording to this aspect of the invention includes a housing defining abore having an axis, one or more rotors disposed within the bore, eachrotor including a plurality of magnet poles, and one or more statorssurrounding the bore for providing a rotating magnetic field within thebore to induce rotation of each of the one or more rotors. The one ormore rotors may be constructed and arranged so that during operation ofthe pump the one or more rotors are suspended within the bore of thehousing and out of contact with the housing solely by forces selectedfrom the group consisting of magnetic and hydrodynamic forces. In thisembodiment the pump has a maximum lateral dimension in any directionperpendicular to the axis of the bore, or a diameter of up to about 20mm. In one embodiment the diameter of the bore is about 14 mm. Inanother embodiment the diameter of the bore is between 9 and 11 mm. Thepump of the present invention can impel from about 1-3 liters of bloodper minute. In one embodiment the pump is adapted to impel about 2liters of blood per minute. Blood pressure can be maintained within therange of from 70-120 mm Hg between the inlet and outlet. The pump isadapted for positioning within an artery, and may include a gripperadapted to engage the wall of an artery.

Another aspect of the invention includes methods of providing cardiacassistance to a mammalian subject as, far example, a human. Methodsaccording to this aspect of the invention include advancing a pumpincluding a housing having a bore, one or more rotors disposed withinthe bore and one or more motor stators disposed outside of the housingthrough the vascular system of the subject until the pump is disposed atan operative position at least partially within an artery of thesubject, and securing the pump at the operative position. The methodincludes the step of actuating the pump to spin the one or more rotorsand pump blood distally within the artery solely by applying electricalcurrents to the one or more motor stators and to suspend the one or morerotors within the bore solely by forces selected from the groupconsisting of magnetic and hydrodynamic forces applied to the one ormore rotors.

Still further aspects of the present invention include rotor bodieshaving helical channels formed longitudinally along the length of thebody of the rotor. Each helical channel is formed between peripheralsupport surface areas facing substantially radially outwardly andextending generally in circumferential directions around the rotationalaxis of the rotor. Each channel has a generally axial downstreamportion. The helical and axial portions of each of the channelscooperatively define one or more continuous flow paths extending betweenthe upstream and downstream ends of the rotor. In one embodiment, theaxial regions of the channels have greater aggregate cross-sectionalarea than the one or more passages. The support surfaces of the rotorbody are formed on a plurality of lobes. Each lobe has a circumferentialextent which increases in a radially outward direction away from therotational axis of the rotor. The support surfaces face generallyradially outwardly away from the rotor axis and define hydrodynamicbearing surfaces. The circumferential extent of the support surfaces isgreater than the circumferential extent of peripheral surface areas.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic perspective view of a pump in accordance withone embodiment of the present invention, with components omitted forclarity of illustration.

FIG. 2 is a diagrammatic view of components used in the pump of FIG. 1,with certain components depicted as transparent for clarity ofillustration.

FIG. 3 is a partial cut-away view of the pump depicted in FIGS. 1 and 2.

FIG. 4 is a fragmentary sectional view depicting a portion of the pumpshown in FIGS. 1-3.

FIG. 5 is a further fragmentary sectional view depicting another portionof the pump shown in FIGS. 1-3.

FIGS. 6 and 7 are perspective views depicting a rotor used in the pumpof FIGS. 1-5.

FIGS. 8 and 9 are elevational views of the rotor shown in FIGS. 6 and 7.

FIGS. 10-14 are sectional views taken along frames 10-14 respectively inFIG. 9.

FIG. 15 is a fragmentary, diagrammatic sectional view depicting aportion of the rotor depicted in FIGS. 6-14 in conjunction with anothercomponent of the pump.

FIG. 16 is a diagrammatic perspective view depicting another rotorutilized in the pump of FIG. 1.

FIG. 17 is a diagrammatic elevational view of the rotor shown in FIG.16.

FIG. 18 is a diagrammatic perspective view of the pump depicted in FIGS.1-17 in conjunction with a further component.

FIG. 19 is a partially blocked diagrammatic view depicting the pump ofFIGS. 1-17 in operating position in the cardiovascular system of asubject.

FIGS. 20 and 21 are diagrammatic perspective views depicting componentsused in further embodiments of the invention.

FIG. 22 is a diagrammatic perspective view of a rotor according to afurther embodiment of the invention.

FIG. 23 is a diagrammatic sectional view taking along line 23-23 in FIG.22.

FIG. 24 is a diagrammatic elevational view depicting a rotor accordingto yet another embodiment of the invention.

FIG. 25 is a diagrammatic elevational view depicting a component of astator used in a pump of FIGS. 1-17.

FIG. 26 is an electrical schematic diagram of the stator used in thepump of FIGS. 1-17.

DETAILED DESCRIPTION

A pump 10 in accordance with one embodiment of the invention includes ahousing 12 (FIGS. 1, 2 and 3). Housing 12 is a ceramic tube defining acentral bore 14 having an axis 16. Bore 14 is cylindrical and has aconstant diameter over the major portion of its length. The interiorsurface 13 of the housing defining bore 14 is smooth, and desirably hasa surface roughness on the order of 4 micro inches rms or less. Merelyby way of example, the inside diameter of bore 14 in this constantdiameter region may be about 0.178 inches (“in”), and the wall thicknessof the housing may be about 0.010 in. Housing 12 defines an inlet 18 atan end 20 of the housing, referred to herein as the inlet or upstreamend, and an outlet 22 communicating with bore 14 at an output ordownstream end 24 of the housing. The inside diameter of inlet 18 isslightly less than the inside diameter of bore 14. The housing includesan inlet transition section 26 having an inside diameter which increasesprogressively in the downstream direction at the juncture between inlet18 and bore 14. The inside diameter of the housing increasesprogressively at an outlet transition section 23 immediately upstreamfrom outlet 22.

As best seen in FIG. 4, a thin-walled metallic tube 28 is fitted overthe inlet or upstream end of the housing so that the interior of tube 28communicates with inlet opening 19. Tube 28 may be formed from a metalsuch as titanium, titanium alloy, or platinum, and may be brazed to theceramic housing. An upstream end fitting 30 surrounds tube 28, and alsosurrounds the upstream end 20 of the ceramic housing 12. A flexibleintake tube 32 surrounds the upstream end of fitting 30 and tube 28, andis held in place by a crimp metal band 34. As best seen in FIG. 1,intake tube 32 extends upstream from fitting 30 and from housing 12, andterminates at a castellated opening 36 at its upstream end. As best seenin FIG. 4, the interior of tube 32 communicates with inlet 18 of housing12, and thus with the bore 14 of the housing, through tube 28. In oneembodiment, intake tube 32 is formed from a non-thrombogenic flexiblepolymer such as, for example, a fluoropolymer, polydimethylsiloxane,silicone polycarbonate urethanes, thermoplastic polyurethanes,polycarbonate urethanes, segmented polyurethanes,poly(styrene-b-isobutylene-b-styrene, or sulfonated styrene containingcopolymers.

A metallic outflow tube 40 (FIG. 5) surrounds the downstream end 24 ofhousing 12 and communicates with the bore 14 of the housing. Outflowtube 40 may be formed from materials as discussed above with respect totube 28. The downstream end of outflow tube 40 defines the outlet 41 ofthe pump 10.

A downstream end fitting 42 surrounds outflow tube and the downstreamend 24 of housing 12. An elongated electrical cable 44, of which only aportion is shown in FIGS. 1 and 5, is secured to downstream end fitting42. As best seen in FIG. 1, the downstream end fitting 42 carriesseveral miniature electrical feedthroughs 46, which are electricallyisolated from one another and which are connected to the individualconductors of cable 44.

A first stator 48 surrounds housing 12 adjacent the upstream endthereof, shown in FIG. 25. The stator 48 includes a magneticallypermeable frame 700. The frame is configured as a cylindrical, tubularring 702 having a plurality of poles 704 projecting inwardly from thering 702 to the exterior of housing 12; six poles are used in thisparticular embodiment depicted. Ring portion 702 is concentric withhousing 12 and bore axis 16. Each pole includes a widened portion at thetip of the pole, where the pole confronts the exterior surface ofhousing 12. The poles define six slots, 706-1 through 706-5, betweenthem. The dimensions of the stator are substantially uniform along theaxial length of the stator. En this embodiment the stator is formed fromnumerous uniform laminations stacked on one another. The laminations areformed from a magnetically permeable material selected to minimize powerlosses due to magnetic hysteresis. For example, the laminations may beformed from 29-guage silicon steel of the type sold under thedesignation M15 electrical steel.

In the particular embodiment depicted, the exterior diameter OD of ringportion 702 is about 0.395 inches, and the interior diameter ID of thering portion is about 0.304 inches. The width or circumferential extentPW of each pole is about 0.035 inches at its juncture with the ringportion 702. The interior diameter PD between opposed pole tips may beabout 0.221 inches. The axial length of the frame is selected accordingto desired output power, and may be, for example, about 0.35 inches forabout 1 watt output to about 0.85 inches for about 3 watts output.

Stator 48 further includes coils 710, 712, and 714 shown in electricalschematic in FIG. 26. In the particular embodiment depicted, each coilincludes about 11 to about 14 turns of 33- or 34-guage insulated wire,and may be impregnated with a material such as a varnish after winding.The coils are connected in a WYE configuration to a common neutral 718.Coils 710, 712, and 714 are disposed respectively in slots 706-1 through706-6 of frame 700 (FIG. 25). Coil 710 is wound through slots 706-1 and706-4, whereas coil 712 is wound through slots 706-3 and 706-6, and coil714 is wound through slots 706-5 and 706-2. This arrangement is commonlyreferred to as a balanced three-phase integral slot winding. The ends ofthe coils remote from the neutral point 718 are connected to inputs P1,P2, and P3. When these inputs are energized with three sinusoidalvoltages offset from one another in phase by 60°, the coils provide amagnetic field within the bore which is directed transverse to the boreaxis 16 and which rotates around the axis, within a first region 50(FIGS. 1 and 2) of the bore. A second stator 52 is disposed downstreamfrom first stator 48, and is arranged to apply a rotating magnetic fieldwithin a second region 54 of bore 14 downstream from the first region.The second stator may be similar to the first stator. Pump 10 furtherincludes a casing 47, depicted in broken lines in FIG. 1, extendingbetween the outflow fitting 42 and the inflow fitting 30, and coveringthe stators 48 and 52. A potting material (not shown) fills space withincasing 47 around the stators.

A first or upstream rotor 55 is disposed within bore 14 adjacent theupstream end of bore, within region the first region 50. A second rotor58 is disposed within the bore downstream from first rotor 56, withinregion 54.

The first rotor 56, shown in FIGS. 2 and 6-15, is formed as a solid,unitary body of a ferromagnetic, biocompatible material, such as analloy including platinum and cobalt, as, for example, an alloyconsisting essentially of platinum and cobalt such as 77.3% Pt and 22.7%Co. The rotor has an upstream or inlet end 60, a downstream or outletend 62, and a rotational axis 64 depicted in dotted line in FIG. 6. Therotor includes a unitary central shaft portion 66 immediatelysurrounding the axis and coaxial therewith, extending throughout thelength of the rotor. The central shaft portion has a generally sphericaldome 67 at its upstream end and a conical, tapered region 65 at thedownstream end 62.

The body of rotor 56 is described herein with reference to axis 64. Asused herein with reference to a structure such as rotor having upstreamand downstream ends and an axis, the upstream direction is the directionparallel to the axis toward the upstream end, whereas the downstreamdirection is the opposite direction. A “radial” direction is a directionoutwardly, away from the axis. A “circumferential” direction is adirection around an arc in a plane perpendicular to axis 64. The“forward” circumferential direction indicated by one end F of the arrowFR in FIG. 6 corresponds to the direction of rotation of rotor 56 aboutaxis 64 in service. The opposite circumferential direction indicated byone end R of the arrow FR (FIG. 7) is referred to herein as the reversecircumferential direction.

A region 68 of the rotor adjacent the upstream end 60, referred toherein as the “helix” region, rotor 56 has helical channels 74 and 76defining a pair of raised peripheral surface areas 70 and 72, alsoreferred to herein as vanes 70 and 72, radially outwardly from thecentral shaft 66. The channel 74 defines a surface area 78 facing in theforward circumferential direction, referred to herein as the pressuresurface or, alternatively referred to as the leading surface of vane 70.Pressure surface 78 is a helical surface of variable pitch along theaxial length. The pressure surface has a pitch angle A (FIG. 8) of about60°. As used in this disclosure with reference to a helical surface, theterm “pitch angle” refers to the angle between the axis of revolution,axis 64, and a line tangent to the surface. The channel 76 defines asurface area 80, facing in the rearward circumferential direction,referred to herein as the suction surface or alternatively referred toas the trailing surface of vane 70. Suction surface 80 is also helical,but has a slightly larger pitch angle than pressure surface 78, so thatthe thickness or circumferential extent of raised surface area of vane70 increases progressively in the downstream direction. (to the right asseen in FIG. 9). The suction surface 80 is truncated by a small flatsurface 83 in a plane perpendicular to the axis 64 at the upstream endof the helix region, thereby defining a sharp, radially-extensive edge85, having a radius of about 0.003 inches, at the upstream end of thehelix region. The raised peripheral surface area of vane 70 is arcuateand of constant radius about axis 64. The surface area of vane 70extends through about 130° of arc about axis 64 from its upstream edgeto its downstream edge.

Surface area of vane 72 is identical to surface area of vane 70, and isoffset from surface area 70 by 180° about axis 64. As best appreciatedwith reference to the cross-sectional view of FIG. 10, the surface areas70 and 72 are thin. The circumferential extent of surface area of vane70 is about 15° of arc around axis 64, and thus the aggregatecircumferential extent of surface areas of vanes 70 and 72 amounts toabout 30° of arc.

As used in this disclosure, the term “major diameter” of a body havingan axis refers to the dimension which is twice the greatest radius fromthe axis to any point on the body in a particular plane perpendicular tothe axis. For rotor 56, the major diameter is simply the length of aline extending between the vanes 70 and 72 through axis 64. As used inthis disclosure, the term “solidity” refers to the ratio between thecross-sectional area of the solid features of the body to the area of acircle having a diameter equal to the major diameter of the body. Thesolidity of the helix portion 68 is in the range of about 10-20% at theupstream or inlet end of the body. In one embodiment the solidity is inthe range of about 10-15%, and about 14%, and increases progressively toabout 15-25% at the downstream end of the helix region, to about 18-23%.In one embodiment the solidity is about 20%. Stated another way, thehelix region is largely open for entry of blood at its upstream end.

The rotor 56 further includes a support region 88 (FIG. 6) disposeddownstream from the helix region 68. The rotor has a first lobe 90 andsecond lobe 92 projecting outwardly from central shaft 66 in the supportregion. First lobe 90 has a pressure surface 94 (FIG. 9), also referredto as a leading surface, facing in the forward circumferentialdirection. Pressure surface 94 is continuous with the pressure surface78 of vane 70. First lobe 90 also has a suction surface 96 (FIG. 6),also referred to as a trailing surface. Suction surface 96 is continuouswith the suction surface 80 of vane 70. Thus, the periphery of the firstlobe 90 constitutes a continuation of the peripheral surface area ofvane 70 in the downstream direction. The first lobe also has a surface98 (FIG. 13) facing generally radially outwardly away from axis 64, thissurface being referred to herein as a “support” surface. The oppositelobe 92 has a similar pressure surface 99 continuous with the pressuresurface at area of vane 72, and suction surface 100 (FIG. 9) continuouswith the suction surface at area of vane 72. Lobes 90 and 92 arediametrically opposite to one another and define passages 106 and 108between them. Passage 106 is continuous with channel 74, whereas passage108 is continuous with channel 76. The passages extend to the downstreamend of the body and are open at the downstream end, so that the channelsof the helix region and the passages of the support region cooperativelydefine continuous flow paths extending between the upstream anddownstream ends of the body. The pressure and suction surfaces of thelobes have substantially constant pitch angle, and the pitch angles ofthe pressure and suction surfaces are substantially equal to oneanother, so that the circumferential extent of each lobe remainssubstantially constant throughout the support region 88. The pitch angleof the pressure and suction surfaces of the lobes are substantiallysmaller than the pitch angle of the pressure and suction surfaces in thehelix regions. For example, the pitch angles of the lobe pressure andsuction surfaces may be on the order of about 10°.

The major diameter of the support section defined by the lobes is equalto the major diameter of helix section. However, as best appreciated bycomparison of FIG. 13 with FIG. 10, the circumferential extent ofsupport surfaces 98 and 104 of the lobes is much greater than thecircumferential extent of the peripheral edge surfaces 82 and 84 of thehelix region. For example, each support surface may have acircumferential extent of about 90-110° of arc about axis 64, so thatthe aggregate circumferential extent of the support surfaces is about180-220°. Moreover, the solidity of the support region including thelobes is substantially greater than the solidity of the helix region.The solidity of the support region may be about 30-40%.

As also apparent from FIGS. 13 and 14, the pressure surface 94 andsuction surface 96 of lobe 90 diverge from one another in the radiallyoutward direction, away from axis 64. Similarly, the pressure surface 99and suction surface 100 of lobe 92 diverge from one another in theradially outward direction. Stated another way, the circumferentialextent of each lobe increases progressively in the radially outwarddirection, so that the mass of each lobe is concentrated in the regionof the love remote from axis 64.

Support surface 104 of lobe 92 has a trailing land area 110 (FIGS. 8 and9) disposed at the same radius from axis as the peripheral surfaces ofthe helix regions. The trailing land area 110 extends along the trailingor suction edge of support surface 104, i.e., the edge of the supportsurface at its juncture with the suction surface 100 of lobe 92. Landarea 110 merges into the peripheral surface 84 of helix area 72, as bestseen in FIG. 8. The support surface 104 further includes a first orupstream hydrodynamic-bearing surface 112 and a separating land 114extending in the forward circumferential direction from trailing edgeland 110 to the forward edge of the support surface, at its juncturewith pressure surface 99. Land 114 lies at the same radius from axis 64as the trailing edge land 110. Thus, bearing surface 112 is bounded onits trailing and upstream sides by trailing edge land surface 110 andperipheral surface 84, and on its downstream side by separating land114. As best seen in FIG. 15, bearing surface 112 is disposed radiallyinwardly from the land surfaces and slopes radially outwardly towardsits trailing edge, i.e., in the reverse circumferential direction towardtrailing land surface 110. Stated another way, the land surfaces definea generally cylindrical surface at the major diameter, and bearingsurface 112 defines a depression in this generally cylindrical surfacewhich tapers to a decreasing depth in the reverse circumferentialdirection. As also shown in FIG. 15, the major diameter of the rotordefined by land surfaces 110 is just slightly less than the internaldiameter of bore 14 in the housing. For example, the internal diameterof the bore may be about 0.002 inches larger than the major diameter ofthe rotor.

Support surface 104 further includes a second or downstream bearingsurface 116 immediately downstream from separating land 114, and adownstream end land surface 118 immediately downstream of the bearingsurface 116. Bearing surface 116 is configured in the same way asbearing surface 112, and forms a similar depression in the cylindricalouter surface tapering to a progressively decreasing depth in thereverse circumferential direction, toward the trailing edge land surface110.

All of the surfaces of rotor 56 are smooth, desirably to a surfaceroughness of about 4 micro inches or less. Rotor 56 may be formed, forexample, by machining from a solid rod and polishing using techniquessuch as electropolishing and drag polishing. Rotor 56 has a permanentmagnetization with a flux direction transverse to axis 64, so that lobe92 forms one pole of a permanent magnet, where lobe 90 forms theopposite pole.

Rotor 56 may have an axial length, from the upstream edges of the helixareas to the downstream end of the lobes of about 0.5-0.95 inches,preferably 0.6 inches long. The helix region may be about 0.15-0.25inches long, preferably about 0.2 inches long whereas the support regionmay be about 0.35-0.45 inches long in the axial direction, preferablyabout 0.4 inches long. The ratio between the length of support regionand the length of the helix region is about 1:1 to 3:1, preferably about2:1.

The second rotor 58 (FIGS. 16 and 17) is similar to the first rotor 56discussed above. Thus, the second rotor includes a helix region definedby channels 174 and 176 adjacent the upstream or inlet end 160 of therotor. The second rotor has lobes 190 and 192 defining passages betweenthem, the passages between the lobes being continuous with the channelsin the helix region. Lobes 190 and 192 are configured in substantiallythe same way as the lobes of the first rotor discussed above withreference to FIG. 13. Thus, in this rotor as well, each lobe has acircumferential extent which increases in the radially outward directionso as to provide a support surface having a substantial circumferentialextent. Here again, the solidity of the rotor in the support regionoccupied by the lobes is substantially greater than the solidity of therotor in the helix region.

Second rotor 58 has a pitch opposite to the pitch of the first rotor.The forward circumferential direction F′ of second rotor 58 is theclockwise direction of rotation about axis 164 as seen from the upstreamend 160 of the rotor, whereas the forward circumferential direction ofthe first rotor 56 (FIG. 6) is the counterclockwise direction as seenfrom the upstream end 60 of the rotor. Also, the second rotor 58 issubstantially shorter in the axial direction than the first rotor. Theaxial length of the second rotor may be about 0.3-0.5 inches. In oneembodiment the axial length is 0.4 inches. Of this length, approximately0.15 inches is occupied by the helix region, and approximately 0.25 isoccupied by the support section consisting of the lobes 190, 192. Thepitch angle A′ (FIG. 17) of the channel surfaces defining the helixregion is substantially greater than the pitch angle A (FIG. 8) of thecorresponding surfaces in the first rotor. Here again, each channelsurface extends helically around axis 64 by about 130° from the upstreamend to its juncture with the associated lobe.

As best seen in FIG. 18, pump 10 desirably is provided with anexpansible gripper adapted to engage the interior surface of an artery.Unless otherwise stated, dimensions of the pump referred to hereinexclude the gripper. The gripper depicted is a stent 200 which includesa thin-walled cylindrical shell 204 having numerous perforationsextending through it. Stent 200 also includes a central collar 206 andthree legs 208 which extend from the collar to the downstream edge ofshell 204, i.e., the edge of the shell facing upwardly in FIG. 18.Collar 206 is mounted on the outflow end fitting 42 of pump 10 at thedownstream or outlet end of the pump. Power cable 44 projects downstreamthrough the interior of shell 204.

In the expanded condition depicted in FIG. 18, stent 204 is spacedradially outwardly from the pump. Stent 204 has a collapsed condition(not shown) in which the stent is disposed downstream from the outletfitting 42, with legs 208 extending generally parallel to the axis ofthe pump and downstream from the pump. In this collapsed condition,stent 204 has an exterior diameter approximately equal to or smallerthan the exterior diameter of pump 10, i.e., about 13 mm or less. In oneembodiment the stent diameter is about 12 mm.

In one embodiment stent 200 is formed from a shape-memory alloy such asthe alloy sold under the registered trademark Nitinol™. The stent isinitially provided in the collapsed condition, and is arranged to returnspontaneously to the expanded condition when the stent is leftunconstrained and heated to body temperature.

In operation, pump 10 and stent 200 are inserted into the patient'svascular system as, for example, into the femoral artery or anotherartery having good access to the desired placement site, and advancedthrough the vascular system, with the intake tube 32 leading, until thepump is in the desired location. As shown in FIG. 19, the pump may beplaced within the patient's aorta, with intake tube 32 extending throughthe aortic valve 220 of the subject's heart, and with the upstream end36 of the inflow tube protruding into the left ventricle. In thisposition, power cable 44 extends through the aorta.

The step of advancing the pump through the vascular system may beperformed using generally conventional techniques for placement ofintra-arterial devices. For example, an introducer catheter may beplaced using a guidewire; the guidewire may be removed, and then thepump may be advanced through the introducer catheter, whereupon theintroducer catheter is removed. Alternatively, the pump may be providedwith fittings suitable for engaging the guidewire. For example, thestent itself may serve as one such fitting at the downstream or outputend of the pump, whereas the inflow tube 32 may be provided with a hole(not shown) extending through its wall adjacent its upstream end 36, sothat the guidewire is threaded through the interior of the stent andthrough the hole in the intake tube. In this case, the pump is advancedover the guidewire without using an introducer catheter.

Before or after placement of the pump, the end of power cable 44 remotefrom the pump is connected to a control unit 222. The control unit 222,in turn, is connected to a storage battery 224. The control unit andbattery may be provided as a unitary device in a common implantablehousing. Control unit 222 is electrically connected through cable 44 tothe stators 48 and 52 of pump 10 (FIGS. 1 and 3), and is arranged toapply appropriate excitation currents to these stators to providerotating magnetic fields as discussed below. The control unit senses thevoltage on the stators and thus detect back EMF generated by rotation ofthe rotors. The control unit controls the excitations of the stators soas to maintain the rotors at a predetermined speed of rotation. Battery224 is a rechargeable battery, and is connected to a transdermal powerconnection 226 Control unit 222. The battery 224 may be implanted in alocation within the subject's body outside of the vascular system. Thetransdermal power connection may include a coil adapted to draw energyfrom a magnetic field applied through the skin, or may include a fittingextending through the patient's skin and carrying connectors adapted forconductive connection to a power source.

With the pump in place and secured, controller 222 actuates the first orupstream stator to apply magnetic flux within region 50 of housing bore14 (FIG. 2). The flux direction is transverse to the axis 16 of thehousing, and hence transverse to the axis of the first or upstreamstator 48. The controller varies the direction of the fluxprogressively, so that the magnetic field direction rotates about axis16 in the forward direction of first rotor 56. Because of its permanentmagnetization, first rotor 56 aligns itself with the flux direction, andthus spins about its axis at the same rate as the flux. The lobes 90 and92 provide a strong permanent magnet. Moreover, the magnetic material ofthe lobes is concentrated near the outside of the stator, in closeproximity to the wall of bore 14, and thus in close proximity to stator48. This provides a strong magnetic interaction between the stator andthe rotor. In one embodiment the field and rotor rotate within the rangeof about 20,000-60,000 revolutions per minute (rpm) most typically about50,000 rpm.

As the rotor spins about its axis, the bearing surfaces on the lobesadvance with the rotor in the forward circumferential direction. As bestappreciated with reference to FIG. 15, there is a relatively largeclearance (about 0.004 inches) between the interior surface of tube 14and bearing surface 112 at the forward edge, where the surfaceintersects forward surface 99 of the lobe. There is a smaller clearanceat the rearward edge of the bearing surface, near land 110, where thebearing surface transitions smoothly into the land 110. There is an evensmaller clearance between the land 110 and the wall of the housing.Thus, as the bearing surface advances in the forward direction, a highhydrodynamic pressure is created at the rearward portion of the bearingsurface. The same is true for the downstream bearing surface 116 (FIGS.8 and 9), and for the bearing surfaces of the opposite lobe 90. Thehydrodynamic pressures keep the rotor centered in the bore and out ofcontact with the wall of the bore. The land portion 114 between bearingsurfaces forms a barrier to axial flow of blood between the upstreambearing surface 112 and the downstream bearing surface 116. The same istrue for the bearing surfaces of the opposite lobe. This helps to assurethat the bearing surfaces provide independent separating forces ataxially spaced locations along the support region of the rotor, so thatthe rotor resists pitch or yaw of the rotor axis relative to the centralaxis of the bore.

The magnetic field applied by stator 48 maintains the rotor in axialalignment with the stator, and prevents the rotor from moving axiallywithin the bore. Thus, during operation, the rotor is suspended withinthe bore by the hydrodynamic and magnetic forces applied to it, and isentirely out of contact with any solid element of the pump. The rotorthus operates with no wear on the rotor or the housing.

As the first rotor spins, the leading and suction surfaces of thechannels 74 and 76 (FIG. 6) of the first rotor impinge on blood presentwithin the bore 14 of the housing, and impel the blood downstream. Therelatively low solidity provided by the helix region promotes inflow ofblood into the flow channels and thus helps to provide effective pumpingaction. The lobes 90 and 92 provide relatively little pumping action.However, the passages 106 and 108 between the lobes provide a relativelylow-resistance flow path from the channels in the helix region 68 (FIG.6) to the downstream end of the first rotor. As discussed above, thesupport region 88 and lobes 90 and 92 serve to provide support for therotor within the bore and to provide an effective drive action.Surprisingly, it has been found that varying the configuration andsolidity of the rotors along their axial extent, so that the helicalregion exhibits relatively low solidity and relatively small peripheralsurfaces and the lobes have relatively high solidity and substantialcircumferential surfaces, provides a particularly good combination ofpumping action with adequate support and adequate magnetic linkage tothe rotating flux of the stator.

As the upstream rotor 56 spins about its axis, viscous drag exerted bythe rotor and the blood entrained therewith on the blood immediatelyupstream of the rotor within bore 14 tends to impart a swirling orrotational motion to the blood upstream from the rotor, so that theblood approaching the rotor is already spinning in the forward directionof the rotor. In theory, this effect tends to reduce the pumping actionimparted by the rotor. This effect could be mitigated by providing fixedaxial vanes inside the bore just upstream from the rotor. However, it isbelieved that a significant advantage is obtained by omitting suchvanes, so that the bore immediately upstream from the rotor is anunobstructed surface of revolution about the central axis 16, with noobstruction to swirling flow. In one embodiment the unobstructed boreextends upstream from the rotor for at least about 2 times the borediameter. Leaving the bore unobstructed in this manner provides agentler action at the upstream end of the rotor and thus tends to reducehemolysis. Stated another way, limitations on rotor speed which may beimposed by hemolysis considerations are relaxed by providing such anunobstructed bore upstream from the rotor.

All of the surfaces of the rotor and the interior surface of the housingin the vicinity of the rotor are continually washed by flowing blood, sothat there no stasis or pooling of blood. This substantially mitigatesthe risk of thrombus formation. Moreover, because the rotor operateswithout wear on the rotor or the housing, the surfaces of the rotor andhousing remain smooth, which further reduces thrombogenesis. The rotorsconstitute the only parts of the pump which move during operation. Asthe rotors are maintained out of contact with other parts of the pump,the pump has no moving parts which contact one another during operation.In particular, the pump has no seals which contact moving parts duringoperation. A pump without such seals can be referred to as a “seal-less”pump.

Because rotor 56 is a simple, two-pole magnet, the stator need provideonly two flux reversals per revolution. Each flux reversal requires thatthe control unit and battery overcome the inductive impedance of thestator, and each flux reversal consumes power in hysteresis of theferromagnetic material in the stator. Accordingly, the frequency ofmotor commutation required for a given rotational speed is lower for atwo-pole rotor than for rotors with a greater number of poles.

The second or downstream rotor 58 operates in substantially the samemanner as the first rotor 56, and is suspended within bore 14 of housing12 by a similar combination of magnetic and hydrodynamic forces. Thesecond rotor spins in the opposite direction from the first rotor. Theblood passing downstream from the upstream rotor 56 has a swirlingmotion in the forward or rotational direction of the first rotor, i.e.,in the direction opposite to the direction of rotation of the secondrotor. In the particular embodiment illustrated, the downstream orsecond rotor provides approximately a third of the pumping workperformed on the blood passing through the pump, whereas the upstreamrotor provides approximately two-thirds of the pumping work. As themagnetic fields associated with each stator apply torque to the rotorsto turn the rotors about their axes, an equal but opposite torque isapplied to the stators. Because the rotors turn in opposite directions,these reaction torques applied to the two stators tend to counteract oneanother, and thus reduce the torque load applied to stent 200 (FIG. 19).

Pump 10 in the embodiment described can pump approximately 3 liters perminute against a pressure differential of approximately 100 mm Hg, atypical physiological blood pressure. The pump thus provides substantialassistance to the pumping action applied by the left ventricle of theheart. Moreover, the pump provides this effective pumping assistance ina device that is small enough to be implanted in the aorta using aminimally invasive procedure, and which can operate for extended periodswithout wear or mechanical failure.

The leaflets of the patient's aortic valve 220 (FIG. 19) seal againstthe exterior surface of inflow tube 32, and thus prevent backflow ofblood into the left ventricle during diastole. When the ventriclecontracts, during systole, blood pumped by the heart flows through theaortic valve around the inflow tube and the pump. The pump and the stentdo not occlude the aorta, and do not prevent the heart from exerting itsnormal pumping action. Thus, in the unlikely event of a pump failure,the patient's heart can continue to provide some blood circulation.Depending upon the patient's condition, this circulation may be adequateto sustain life until corrective action can be taken.

Numerous variations and combinations of the features described above canbe utilized without departing from the present invention. For example,the second or downstream rotor and the corresponding stator may beomitted to provide a smaller pump with somewhat lesser pumping capacity.Conversely, three or more rotors may be utilized. The dimensions andproportions discussed above can be varied. For example, the housing,rotors and stators can be made with a substantially larger diameter toprovide more pumping capacity in a pump which is to be implantedsurgically, as for example, by connection through the apex of the heart.

The pump can be positioned in other locations. In one such variant, theintake tube is omitted and the pump is positioned proximally from theposition illustrated in FIG. 19, with the inlet end of the pump housingitself protruding through the aortic valve. In yet another variant, thepump may be positioned in the descending aorta, in the femoral artery orin another artery, so as to provide localized circulatory assistance.The pump also may be implanted into a pulmonary artery to provideassistance to the right ventricle.

A pump 310 in accordance with a further embodiment (FIG. 20) includes agripper 300 having a collar 302 attached to the outflow fitting orshroud of the pump adjacent the downstream or outlet end of the pump,and a set of fingers 304 attached to collar 302 at locations spacedapart from one another circumferentially around the axis of the pump.Each finger has a tip 306 remote from the collar and a crown sectionhaving a relatively large circumferential extent disposed near the tip.Each finger also includes a beam section 311 between the crown section308 and collar 302. In the expanded condition illustrated in FIG. 20,the beam sections 311 project outwardly from the collar 302, and thecrown sections have a curvature so that the tips 306 point inwardlytoward the axis of the pump. The crown sections 308 bear en the interiorwall of an artery (not shown), whereas the tips 306 are maintained outof contact with the artery wall. The beam sections 311 have relativelylow resistance to flow of blood in the axial direction. Fingers 304 maybe formed from a shape memory alloy as discussed above, and have acollapsed condition in which they project axially and thus lie flatagainst the exterior surfaces of pump 310. In a variant, the tips 306may project inwardly toward one another at the upstream or inlet end ofthe pump to facilitate movement of the pump along the artery duringplacement.

Referring to FIG. 21, a pump 410 according to yet another embodiment ofthe invention includes a gripper in the form of two strips 402 and 406.In the collapsed condition, strip 406 is wound in a helix, closelyoverlying the exterior surface of the pump. The downstream end of thestrip 406, closest to the downstream end 412 of the pump, is affixed tothe pump body. The upstream end 414 is free, and is secured to the pumponly by the other turns of the helix. The second strip 402 has itsupstream end 409 affixed to the pump body, and its downstream end 408free to move relative to the pump body, constrained only by theremaining turns of the helix. When the pump is implanted, strip 402assumes the expanded position shown at 402′. In this condition, strip402 is generally in the form of a spiral extending clockwise about theaxis of pump 410, as seen from the downstream end 412 of the pump. Strip406 assumes the shape shown at 406′ and approximates a spiral with theopposite direction from spiral 402′, i.e., extending counter-clockwisefrom its juncture with the pump body to its free end 414, again as seenfrom the downstream end 412 of the pump.

Grippers described herein can be configured at intervals along adriveline that extends from the pump to a battery or a controller. Suchdriveline is downstream of the pump and a configuration of grippersalong its length can be used to maintain driveline position in a mainpath of blood flow end away from arterial walls. The combination ofgripper support of the pump and gripper support of the driveline easesthe removability of the pump if, for example, repair is needed or thepump is no longer needed by the patient. Driveline gripper supports maybe attached to the driveline in intervals of roughly 0.23-0.46 inchesalong the length of the driveline.

As discussed above, the proportions of the rotors can be varied. Morethan two lobes and helix sections may be employed. In one embodiment,the number of lobes is equal to the number of helix sections. However,other configurations can be employed. Also, the rotors discussed abovehave the same major diameter over the length of the pump body so that,considering the major diameter only, the rotor is generally cylindrical.This also is not essential. For example, a rotor may have a helixsection with a greater major diameter than the support section. Such arotor may be used with a tapered housing. The stator may surround onlythe region of the housing which receives the support region, so that thepump as a whole has a small diameter.

Merely by way of example, a rotor according to a further embodiment ofthe invention (FIGS. 22 and 23) includes a helix section 568 asdiscussed above, but includes a support section 588 in the form of ahollow tubular shell having a single interior bore 502, best seen inFIG. 23, which depicts a cross section view looking from the downstreamend of the rotor. The shell also defines a support surface in the formof a cylinder with a full 360° extent around the axis 564 of the rotor.The channels 574 and 576 of the helix section communicate with passage502 of the support section through ports 506 at the juncture between thesupport section and the helix section. The particular configuration ofthe ports 506 in FIGS. 22 and 23 is schematic. Desirably, the portswould have surfaces merging smoothly with the channels defining thehelix section. The cylindrical support surface can act as a hydrodynamicbearing surface. To assure continuous washing of the support surface 504and the interior bore of the housing, holes 508 may be provided throughthe wall of the tubular support section.

Referring to FIG. 24, a rotor 600 according to a further embodiment ofthe invention includes a helix region 602 with peripheral surface areas606 and channels 608 similar to those discussed above. The rotor alsoincludes a support region 604. In this embodiment, the support regionincludes an upstream portion 610, a downstream portion 620 spaced-apartfrom the upstream portion in the axial direction and a shaft 630extending between these portions. Upstream portion 610 has lobes 614 andchannels 616 between the lobes. Here again, lobes 614 have radiallyoutwardly facing support surfaces which incorporate hydrodynamic bearingsurfaces 618. The downstream portion 620 has lobes 624 and passages 625between the lobes. Lobes 624 define further hydrodynamic bearingsurfaces 628. Shaft 630 has a relatively small cross-sectional area.Passages 616 and 626 communicate with the space surrounding the shaft.The small cross-sectional area of the shaft provides a relatively lowresistance to axial fluid flow. The axial spacing of the upstream anddownstream portions from one another provides a greater axial distancebetween the hydrodynamic bearing surfaces, and thus provides greaterresistance to yaw or tilting of the rotor in directions transverse tothe central axis of the housing.

In the embodiments discussed above, each rotor is formed entirely as aunitary body of a single ferromagnetic material. However, this is notessential. For example, the rotor could be formed from a ferromagneticmaterial such as iron or an iron-nickel alloy, which has desirableferromagnetic properties, but which is far less compatible with theblood. The rotor may be plated with a metal having desirable bloodcompatibility of properties such as platinum, with or without one ormore intermediate plating layers. In yet another variant, the helixsection of each rotor may be formed from a non-ferromagnetic materialwhich is bonded to a support section incorporating a ferromagneticmaterial.

As these and other variations and combinations of the features discussedabove can be utilized, the foregoing description of the preferredembodiments should be taken by way of illustration rather than by way oflimitation of the invention as defined by the claims.

What is claimed is:
 1. A method of providing cardiac assistance to amammalian subject comprising the steps of: (a) advancing: a pumpincluding a housing having a bore, one or more rotors disposed withinthe bore and one or more stators disposed outside of the housing; adriveline coupled to the pump; and a plurality of grippers coupled tothe pump and extending along the driveline through a vascular system ofthe subject until the pump is disposed at an operative position at leastpartially within an artery of the subject; (b) securing the pump at theoperative position at least partially by actuating the plurality ofgrippers to engage the artery of the subject; and (c) actuating the pumpto spin the one or more rotors and pump blood distally within the arterysolely by applying electrical currents to the one or more stators and tosuspend the one or more stators within the bore solely by forcesselected from the group consisting of magnetic and hydrodynamic forcesapplied to the one or more rotors.
 2. A method as claimed in claim 1,wherein the artery is an aorta of the subject.
 3. A method as claimed inclaim 2, wherein the housing has an inlet and an outlet and theadvancing step is performed so as to place the inlet in fluidcommunication with a left ventricle of the subject's heart and place theoutlet within the subject's aorta.
 4. A method as claimed in claim 3,wherein the pump includes an intake tube and the advancing and securingsteps are performed so as to place the intake tube through an aorticvalve of the subject and position the pump entirely within the aortawith the inlet of the housing communicating with the left ventriclethrough the intake tube.